1. Field of the Invention
This invention relates to advanced lithographic masks. Particularly, it relates to projection lithography employing soft x-ray radiation or extreme ultraviolet (EUV) radiation and, more particularly, to methods and systems that provide for the reduction of thermal gradients on extreme ultraviolet lithography (EUVL) reticles during scanning.
2. Background of the Invention
Extreme ultraviolet lithography is a leading candidate for the production of integrated circuits with feature sizes of 65 nm and below. EUVL is different from other lithographic technologies in that it is based on reflective lithographic technologies when compared to conventional transmissive optical lithography. In a simplified description of this technology, a patterned EUVL mask is exposed to EUV or soft x-ray radiation and the corresponding pattern is eventually reflected rather than transmitted onto a resist coated substrate, where the resist is exposed and the desired pattern is formed.
An EUVL mask consists of an absorbing film deposited on a multilayer reflective coating which is deposited on a substrate. Additional films such as a buffer layer or etch stop layer can be deposited between the multilayer stack and absorber film to aid in certain aspects of the mask manufacturing process if desired. The substrate is typically a material that has very low thermal expansion characteristics. The multilayer reflective coating is a Bragg mirror consisting of 40 bilayers of silicon and molybdenum. The use of silicon and molybdenum as the bilayers is specifically optimized for a peak EUV reflectivity wavelength of 13.4 nm. Other combinations, such as beryllium and silicon, can be used as well, although the peak reflectivity wavelength may differ. The absorber film is any element, compound, or alloy, such as chrome or tantalum nitride, deposited at a thickness such that over 99% of the EUV radiation is absorbed. To create a patterned mask, the absorber layer, as well as any underlying layers between the multilayer stack and absorber layer, are removed in specific areas by known manufacturing processes to create a desired pattern. Where the absorber and underlying layers are removed (the patterned area), the EUV radiation is reflected by the multilayer stack. Where the absorber remains (the unpatterned area), the EUV radiation is absorbed.
The entire mask is not exposed all at once, but the EUVL reticle is scanned across a segment of EUV radiation which exposes only a small portion of the mask, leaving the remainder of the mask unexposed. The radiation exposes both patterned and unpatterned areas.
The absorption of the radiation in the unpatterned areas can lead to noticeable temperature gradients on areas of the mask, which can produce mask distortion due to thermal expansion and consequently image size and placement errors. Modeling results from the University of Wisconsin have shown that thermal gradients of up to 1.9Â° C. can exist across an EUV mask during continuous exposure. For individual points on the mask, the simulation results indicated that temperature variations up 0.48Â° C. can exist due to the localized heating and cooling during the exposure. These results also indicated that in-plane displacements (IPD) can be up to 1.33 nm. Magnification correction has been shown to reduce the IPD's. However, the cyclic heating and cooling during the exposure reduced its effectiveness as did changes in the pattern density. After magnification correction, the maximum IPD was simulated to be 0.6 nm. (See C. Martin, R. Englestad, E. Lovell, “Thermomechanical Modelling of an EUV Reticle During Exposure,” Computational Mechanics Center, University of Wisconsin—Madison, Mar. 7, 2001.) Since these are simulation results, the trends of the results are more important rather than the actual numbers. Because the simulations are based on ideal assumptions and conditions, the actual values are expected to be somewhat higher.
Others have identified thermal gradients as a problem for EUV mask operation and have tried to solve it using substrate cooling or heating systems or processes. For example, European Patent Application EP 1 120 690 A2 and U.S. Pat. No. 6,098,408 issued to Levinson et al. provide such processes. These methods, however, can not account for various pattern densities across a mask. Others have attempted to solve this problem by selective placement or omission of coating on the reticle, for example, U.S. Pat. No. 6,316,150 issued to Gianoulakis et al.